The present invention relates to laser noise removal in an optical disk apparatus using various optical disks such as a DVD (Digital Versatile Disk) and a CD (Compact Disk) as a recording medium, for example.
Playback apparatus as well as recording and playback apparatus using an optical disk as a recording medium have come into wide use. For example, DVD players and CD players are widely used in ordinary households. Optical disk recording and playback apparatus capable of recording on a DVD-R (DVD Recordable), a DVD-RW (DVD Rewritable), a CD-R (CD Recordable), a CD-RW (CD Rewritable) and the like are widely used as a data recording apparatus for a personal computer, for example.
In such an optical disk apparatus as an optical disk playback apparatus or an optical disk recording and playback apparatus, a reproduced radio-frequency signal (reproduced RF signal) obtained from an optical disk includes so-called laser noise caused by laser light applied to the optical disk. Therefore, for highly accurate reproduction, methods for removing the laser noise from the reproduced RF signal have been proposed.
For example, Japanese Patent Laid-Open No. Hei 10-124919 proposes a method for removing the laser noise from the reproduced RF signal by subtracting a monitoring output signal for APC (Automatic Power Control) from the reproduced RF signal. An optical disk playback apparatus using this method is shown in FIG. 16.
As shown in FIG. 16, laser light is applied from a laser light source (semiconductor laser device) 1 through a beam splitter 2 to an optical disk 100. The light reflected from the optical disk 100 is supplied to a photodetector (light receiving device) 3 via the beam splitter 2, and then converted into an electric signal by the photodetector 3 The output signal from the light receiving device 3 is amplified by an amplifier 4, and then supplied to an arithmetic unit (adder) 5 as a reproduced RF signal rf(t).
In the meantime, the laser light emitted from the laser light source 1 is also supplied to a photodetector (light receiving device) 6 via the beam splitter 2, and then converted into an electric signal by the photodetector 6. The output signal from the light receiving device 6 is an APC monitoring output signal used for automatically adjusting power (light intensity) of the laser light. The APC monitoring output signal is supplied to a power control signal generating unit 7. The APC monitoring output signal is also supplied to an amplifier 8 having a gain adjusting function, then adjusted in gain by the amplifier 8, and supplied to the arithmetic unit 5 as an APC monitoring output signal m(t).
The arithmetic unit 5 subtracts the APC monitoring output signal m(t) from the reproduced RF signal rf(t) (antiphase addition). Thus, a so-called additive laser noise component (additive noise component) possessed by the laser light itself is subtracted and hence removed from the reproduced RF signal rf(t) including the laser noise component.
The reproduced RF signal rf(t) from which the additive noise component is removed is supplied to an equalizer 9 to be subjected to predetermined equalizing processing. Thereafter, a data discriminating unit 10 discriminates (distinguishes) data of the reproduced RF signal rf(t). The discriminated data is decoded by a decoding unit 12 and then reproduced. A clock reproducing unit 11 reproduces a clock signal used in discriminating the data.
Thus, by removing the additive noise component possessed by the laser light itself from the reproduced RF signal rf(t), it is possible to reduce jitter of the reproduced RF signal rf(t) around zero crossing level (shaking of the reproduced RF signal in a direction of a time axis), and to thereby improve a reproduction margin. Therefore, an optical disk apparatus with a reproducing system performing binary detection can make favorable reproduction.
Although the above-described conventional method for canceling the laser noise can cancel laser noise around zero crossing level, the method cannot cancel laser noise in other parts. The reproduced RF signal rf(t) is formed from the laser light applied to the optical disk and reflected from the optical disk. The light reflected from the optical disk is a product of the laser light applied to the optical disk multiplied by a recorded signal recorded on the optical disk (signal recorded on the optical disk by marks and spaces).
Hence, the reproduced RF signal rf(t) includes not only the so-called additive noise component possessed by the laser light itself but also a modulated noise component resulting from the additive noise component being modulated by the recorded signal of the optical disk.
Thus, even when the APC monitoring output signal m(t) formed from the laser light itself from the laser light source 1 and including only the additive noise component as shown in FIG. 17B is subtracted from the reproduced RF signal rf(t) including the additive noise component and the modulated noise component as shown in FIG. 17A, the laser noise around zero crossing level can be removed but the modulated noise component remains, as shown in FIG. 17C.
The mainstream of reproduced signal processing by a reproducing system of an optical disk apparatus has recently been shifting from the conventional binary detection processing to PRML (Partial Response equalization and Maximum Likelihood detection) signal processing.
In the PRML signal processing, the reproduced RF signal uses multiple decision levels, and a Viterbi detector having multiple reference levels is used. It is therefore desirable that an optical disk apparatus having a reproducing system performing PRML signal processing reduce laser noise not only around zero crossing level of the reproduced RF signal but also around each of the reference levels.
Although the conventional method for canceling laser noise as described with reference to FIG. 16 and FIGS. 17A, 17B, and 17C is effective in the reproducing system that performs the conventional binary detection processing, the conventional method is not adequate for use in the optical disk apparatus performing the PRML reproduced signal processing because the conventional method cannot remove the modulated noise component from the reproduced RF signal rf(t).
In addition, the conventional method for canceling laser noise as described with reference to FIG. 16 and FIGS. 17A, 17B, and 17C is required to adjust the amplitude of laser noise dynamically depending on variation in the reflectivity of the optical disk (medium), variation in the gain of the I/V amplifier, variation in laser power and the like. This amplitude adjustment (gain adjustment) is made by the amplifier 8 having a gain adjusting function shown in FIG. 16.
Thus, the conventional method for canceling laser noise requires an AGC (Automatic Gain Control) circuit, and it is extremely difficult for the AGC circuit to perform proper processing because the foregoing various variation factors in laser noise need to be considered.
It is an object of the present invention to provide an optical disk playback apparatus, an optical disk recording and playback apparatus, and a laser noise canceling circuit applied to an optical disk apparatus that can reliably and readily remove all laser noise components from the reproduced RF signal.
To achieve the above object according to first aspect, there is provided an optical disk playback apparatus including: a generating unit for generating laser light to be applied to an optical disk; a first light receiving unit for receiving the laser light applied from the generating unit to the optical disk and reflected from the optical disk, and converting the reflected light into an electric signal; a second light receiving unit for receiving at least part of the laser light applied from the generating unit to the optical disk, and converting the part of the laser light into an electric signal; a low-pass filter for extracting a direct-current component of the laser light from an output signal of the second light receiving unit; a first multiplying unit for determining a product of an output signal from the first light receiving unit and an output signal from the low-pass filter; a high-pass filter for extracting a laser noise component of the laser light from the output signal of the second light receiving unit; a second multiplying unit for determining a product of the output signal from the first light receiving unit and an output signal from the high-pass filter; and an arithmetic unit for subtracting an output signal of the second multiplying unit from an output signal of the first multiplying unit.
The optical disk playback apparatus according to the first aspect extracts the direct-current component of the laser light from a monitoring output signal of the laser light or the output signal from the second light receiving unit by means of the low-pass filter, and extracts the laser noise component of the laser light from the monitoring output signal of the laser light or the output signal from the second light receiving unit by means of the high-pass filter.
The first multiplying unit multiplies a reproduced RF signal or the output signal from the first light receiving unit by the direct-current component of the laser light from the low-pass filter, thereby forming an output signal including the direct-current component of the laser light, a signal component read from the optical disk, an additive component of laser noise, and a modulated noise component resulting from the additive component of the laser noise being modulated by a recorded signal.
The second multiplying unit multiplies the reproduced RF signal or the output signal from the first light receiving unit by the laser noise component of the laser light from the high-pass filter, thereby forming an output signal including the additive component of the laser noise and the modulated noise component resulting from the additive component of the laser noise being modulated by the recorded signal.
Then, the arithmetic unit subtracts the output signal from the second multiplying unit from the output signal from the first multiplying unit (antiphase addition). By performing such product-sum operation, it is possible to remove from the reproduced RF signal both the additive noise component and the modulated noise component of the laser noise included in the reproduced RF signal, and to thereby perform favorable reproduction processing.
In the optical disk playback apparatus according to the first aspect, a pass band which is allowed to pass through each of the low-pass filter and the high-pass filter may be determined on the basis of a data transfer rate of the optical disk playback apparatus.
The above optical disk playback apparatus sets a reference frequency fc for indicating the pass bands of the low-pass filter and the high-pass filter at an appropriate frequency that prevents reproducing jitter from being aggravated, on the basis of the data transfer rate of a reproducing system of the optical disk playback apparatus. The reference frequency fc is for example {fraction (1/1000)} (one thousandth) of a Nyquist frequency fn or lower, which is xc2xd (one half) of the data transfer rate of the reproducing system.
It is thereby possible to accurately extract the direct-current component of the laser light and the laser noise component included in the laser light, and properly remove (cancel) both the additive component and the modulated component of the laser noise included in the reproduced RF signal.